1. Field of the Invention
The invention relates to the field of power measuring devices and, more particularly, to power measuring devices for measuring A.C. power in an electrical distribution system having three or more wires.
2. Description of the Prior Art
Electrical distribution systems for supplying electricity to consumers are generally characterized by comprising two or more live wires, and may further include a ground return path. Typically, North American electrical distribution systems use a three-wire single-phase arrangement with one wire being neutral or at ground potential and the other two wires being at an elevated potential with respect thereto. Generally speaking, the voltages in the two live wires are 180.degree. out of phase with each other such that the instantaneous peak-to-peak potential difference between the two wires, E.sub.O (for example 240 volts), is twice the potential between a live wire and the neutral or ground. Thus it is possible to drive loads requiring two different supply voltages from the same transmission system. For example some appliances, such as electrical dryers or stoves which consume large amounts of current, are operated more efficiently at the full potential E.sub.O (240 volts), whereas less current-hungry electrical appliances and lighting fixtures can be run at the less hazardous 120 volt potential which exists between one of the live wires and the neutral wire.
In metering the amount of power consumed by loads which are connected to the three-wire distribution circuit it is necessary that the currents be measured in each of the live wires or branches since the current may flow between one live wire and neutral or the other live wire and neutral or in both live wires depending upon the connection of loads to the three-wire system. Thus it is conventional to separately measure the currents in each of the live wires and to multiply these currents times the applied voltage E.sub.O to obtain a measure of instantaneous power or demand (measured in watts). This multiplication can take place by means of a conventional electromechanical induction-type wattmeter mechanism. If the measured power is integrated over time, such as through a conventional clock-type register or electronic accumulator, the result is a measure of energy used by the load over the measurement time interval (in watt-hours, for example).
It is also known to use materials which exhibit a galvanomagnetic effect, such as the so-called Hall-effect, as power measuring devices. As is well-known, when an alternating current is inductively coupled to a magnetic core having a gap in which a plate of galvanomagnetic material is disposed, a flux is induced in the gap which is proportional to the current being measured in the line. If an excitation current is derived from the electrical network's supply voltage, E.sub.O, and is applied to the plate of galvanomagnetic material a voltage will be induced in the galvanomagnetic material which is directly proportional to the product of the excitation current (which is proportional to the supply potential E.sub.O) and the current flowing in the measured wire. Thus the output of the device is a voltage which is directly proportional to the instantaneous power being delivered by the current-carrying wire. The use of such a Hall generator as a watt transducer is shown, for example, in U.S. Pat. No. 3,343,084.
As stated above, in a three-wire transmission system it is necessary to measure the currents flowing in each of the live wires in order that all the power may be accurately measured. When Hall generators are used as watt transducers the same constraint applies: the current in each live wire must be measured and combined in a fashion such as to produce an accurate indication of the amount of power being consumed by one or more loads connected to the transmission system.
One such prior art system is shown in FIG. 1 and utilizes separate Hall generators 1 and 3 which measure the currents in each of the live wires L1 and L2 respectively. The output of each of the Hall generators 1 and 3 is a voltage whose magnitude is directly proportional to the power flowing respectively in wires L1 and L2. The outputs of Hall generators 1 and 3 are applied to a summing circuit 5 whose output is indicative of the total power flowing in the live wires L1 and L2 to one or more loads connected to the transmission system. Such a system is shown in U.S. Pat. No. 3,054,952 wherein an oscillograph is used as the means for summing the power indicative voltages from the Hall generators in each branch of the electrical transmission system. However, such an arrangement is expensive since it requires a pair of Hall generators and a separate summing circuit. The accuracy of such an arrangement is highly dependent on accurate matching of the characteristics of each of the Hall-effect generators.
It is also possible to couple a single Hall generator to both of the live wires in a three-wire single-phase transmission system as shown in FIG. 2. In this arrangement, the magnetic field being induced in the Hall generator 1 by the currents in each of the live wires L1 and L2 is magnetically summed in the magnetic core of the Hall generator. The output voltage of the Hall generator is directly proportional to the total amount of power being transmitted through wires L1 and L2.
However, this arrangement has the drawback that both of the supply wires L1 and L2 must be physically proximate to the Hall generator 1. This is not always possible due to installation and/or physical limitations. For example, electrical service to a home or apartment is generally through a junction box containing sets of fuses or circuit breakers for each branch circuit. The live and neutral wires which come from the electricity supplier's distribution transformer generally are eight gauge or heavier and are encased in a protective sheath. At the point of entering the junction box the two live wires are split and run to two parallel busses to which the fuses or circuit breakers are connected. Because of the physical limitations and the gauge of the live wires it is virtually impossible to place both live wires in proximity to the magnetic circuit of a single Hall generator.
In addition to the above, when used as watt transducers Hall generators suffer from a further defect in that their output signal is a distorted version of the measured current signal. This is due to the well known phenomena that the graph of magnetic induction (B) versus magnetic force (H) in the gap of a magnetic circuit (such as a broken toroid of ferromagnetic material) is not a smooth curve or a straight line but rather a rather non-linear function. Thus the smooth periodic nature of the measured current being applied to the magnetic circuit will be distorted by the characteristic non-linear function of the magnetic circuit about its zero point to produce an output which is a distorted version of the input signal.